(1) Field of the Invention
The present invention relates to the testing of polymer to metal bonds, and more specifically to a method and apparatus for demonstrating the durability of polymer to metal bonds in a cathodic environment by subjecting samples to accelerated cathode reactions that cause cathodic delamination.
(2) Description of the Prior Art
Polymer to metal bonds are increasingly used in vessels and devices designed for use in marine environments. It is vital that these bonds maintain their integrity while exposed to saltwater. Of particular concern is the problem of cathodic delamination, which occurs when the metal hull of a marine vehicle is protected from corrosion by using a “sacrificial” anode system. In a situation where the anode is made of zinc, the reaction is 2Zn→2ZN+2+4e−. The cathodic reaction is O2+2H2O+4e−→4(OH)−. Over a period of several months and years, the hydroxide ions (OH)− become highly concentrated at the cathode, creating a high alkalinity water environment destructive to polymer to metal bonds. In fact, cathodic delamination is the most commonly encountered failure for polymer to metal bonds in the marine environment.
In order to anticipate which polymer to metal bonds are most resistant to cathodic delamination, it is necessary to test the bonds in artificially created marine conditions. One prior art testing method and apparatus places the polymer to metal bond samples inside an open container filled with synthetic ocean water, and drill holes in the unbonded portion of the metal in order to suspend the samples from an electrically conductive rod. The water is heated using a stainless steel immersion heater. Any evaporated water is replaced with water of an approximate conductivity to the electrolyte contained in the open vessel. The emphasis on this testing method and apparatus is on water temperature based on the theory that the diffusion of water controls the degradation rate of polymers and adhesives and therefore elevated temperatures will increase the rate of water permeation through the polymer thereby artificially accelerating the “aging” of the polymer to metal bond. There is no attention to dissolved oxygen or conductivity in this testing method.
Water, however, is not the only reactant that can affect the degradation rate of a bond. Cathodic delamination will not proceed if there is no dissolved oxygen in the water or if there are no electrons supplied to the cathode. In light of this, other prior art testing method and apparatus use a bubbling system in an attempt to maintain the dissolved oxygen in water at elevated temperatures. Such a system, however, is limited to maintaining the dissolved oxygen level that is physically allowable at the particular elevated water temperature.
There is currently no method and test apparatus to artificially accelerate the effect of cathodic delamination on polymer to metal bonds that also maintains the levels of dissolved oxygen in water and the levels of electrical current that would be encountered in the normal course by marine vehicles when the metal hull of the marine vehicle is protected from corrosion by using a “sacrificial” anode system. What is needed is a method and test apparatus that subjects different polymer to metal bonds to artificially accelerated cathodic delamination under typical marine conditions.